I have a 1984 GPZ1100 that sat in the garage for 22 years and is now almost ready for the road again. I remember once taking it to 150 and thinking never again! it was all over the place, so I'm not expecting too much when it goes back on the road, hopefully within the next couple of weeks once the forks come back from re-chroming.
I also managed to but a 1978 Z650 a month ago which was a non runner, managed to get it going in an hour but it needs a new starter clutch which will be next on the list once the GPz is on the road
I've decided rather than spend the best part of 10k to change to a 1250GS to spend the money on bikes i remember growing up in the 70's, so now looking for a CB750K as a project but they are making ridiculous money just now.
Is your Gpz still running injection? It's surprising how many are still running it. I have a couple of old magazine comparison tests from the early eighties and for a short time in the early eighties the Gpz1100 was the king of the road and the fastest thing out of the crate. In a big 8 bike shoot out in Super Bike in the early eighties the big Z acquitted itself well against the opposition until the smaller more powerful water cooled bikes appeared on the scene.
Your z650 and a CB750K will be two cracking machines to enjoy for years to come and both are desirable which should be good for future values. I don't buy old stuff expecting a profit but it's nice when they do increase in value rather than depreciate like new stuff does.
I stripped the carbs on the z650 this afternoon and it looks like a previous owner had a go at the outside carbs but not the middle two. The carbs are removed by pushing the airbox rubber boots back into the airbox but they have become stiff so it turned into a slow job but at least I didn't split any of the rubber. Yet.
I will clean the carbs sometime during next week hopefully and get the bike running next weekend fingers crossed.